This invention pertains to a method of controlling the level of entrained air in hydraulic cement compositions. More particularly, this invention relates to a method wherein certain defoaming agents are used which prevent undesired air entrainment, while still permitting the subsequent entrainment of a controlled amount of air if desired for freeze/thaw durability.
Among the various materials added to hydraulic cement compositions such as Portland cement concretes are those which have been found able to affect the interaction between the water and the cement particles in the composition. For instance chemical additives have been used for some time which act to render the wet hydraulic mix more "plastic" at a given proportion of water to cement in the hydraulic mix, or conversely to allow less water to be used in the mix to obtain a given plasticity. These plasticizing materials are referred to as either "water reducing agents" or superplasticizers. Thus, "water reducing agent" and "superplasticizer" are terms of art which reference the relative water-reducing ability of a material. Water reducing agents provide a water reduction capability of 5% to 12% (ASTM-C494, Type A or Type D), while superplasticizers provide water reduction in excess of 12% (ASTM-C494, Type F or Type G). For brevity and convenience, both the water reducing agents and superplasticizers are collectively referred to herein as plasticizers.
The ability of certain chemical additives to reduce the amount of mix water required to obtain a given plasticity, or "slump" as it is referred to in the art, has led to the valuable utility of these materials as compressive strength-enhancing additives for hydraulic cements. It is well established that other factors being equal, a reduction in the amount of the water employed relative to the cement in the mix (the w/c ratio) will lead to an increase in the extended compressive strength of the hydrated cement compositions as usually measured 28 days after the preparation of the cement mixture. Among the various chemical materials which have been employed as strength-enhancing plasticizers are carbohydrates, such as saccharides and polysaccharides, starch and derivatives thereof such as pregelatinized starch, dextrin, cornsyrup, etc.; naphthalene sulfonic acid-formaldehyde condensate polymers and melamine sulfonate formaldehyde condensate polymers and salts thereof; polyhydroxy polycarboxylic compounds such as tartaric acid and mucic acid; lignosulfonic acid and salts therof, for example, sodium lignosulfonate; and polyethers having grafted polyacrylic acid side chains. These substances are effective as plasticizing, strength enhancing additives since they produce no side effects deleterious to strength, although many of these materials will also act to retard the setting time of the hydraulic cement mixture. These materials therefore are normally used in conjuction with a set accelerator such as calcium chloride or a formate salt to offset the retardation effect in instances where such retardation is not also a desired effect.
Since these additives have the ability to reduce surface tension in water they also often increase the plasticity of wet hydraulic cement mixes by entraining air in the wet mix. The art has long recognized this utility of air entraining plasticizers in applications where large amounts of entrained air may be tolerated in order to obtain high plasticity. The increased air entrained during mixing due to the presence of such plasticizers is normally retained in the final hydrated product, and thus another wide use for surface active materials in the hydraulic cement art has been as air-entraining agents for the hydrated cement product to render it more durable and resistant to freeze-thaw cycles. However, the amount of air entrained by these plasticizers is often deleterious to the compressive strength of the final set product.
Thus, it would be particularly advantageous to be able to control the amount of air entrained depending on the properties desired, i.e. to entrain more air when freeze-thaw resistance is needed, less when maximum compressive strength is required. Using conventional air entraining plasticizers, the level of entrained air is difficult to control, may be unacceptably high at useful dosages, and will vary depending on the dosage and the particular additive used.
It is known to use defoamers in conjuction with air entraining additives such as plasticizers in concrete compositions in order to prevent or decrease the entrainment of air during the mixing of the concrete. However, in mixes containing defoamers, if some air entrainment is desired in order to impart freeze/thaw durability to the set concrete, it is generally not possible to subsequently entrain air into these compositions, even if an air entraining agent is added.